Enhanced CDMA System with Secure Scrambling
In this chapter, we consider robust CDMA system design and capacity analysis under disguised jamming, where the jammer generates a fake signal using the same spreading code, constellation, and pulse shaping filter as that of the authorized signal. First, we analyze the performance of conventional CDMA systems under disguised jamming and show that due to the symmetricity between the authorized signal and the jamming interference, the receiver cannot distinguish the authorized signal from jamming, leading to complete communication failure. Second, we explore effective CDMA system design under disguised jamming. We find that it is possible to combat disguised jamming through careful receiver design, by exploiting the time difference between the authorized signal and the jamming interference. However, this approach only works when the time difference between the authorized signal and the jamming is significant enough. A more reliable approach is to equip CDMA with secure scrambling, where we apply Advanced Encryption Standard (AES) to generate the scrambling codes. This approach performs well even if when the authorized signal and the jamming are perfectly synchronized. Based on the information theory, we show that the AVC corresponding to the conventional CDMA is symmetric and hence results in zero deterministic code capacity. On the other hand, the AVC corresponding to securely scrambled CDMA is not symmetrizable and hence can achieve a positive deterministic code capacity under disguised jamming.
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